Adhesives for laminating vinyl to various substrates

ABSTRACT

Adhesives suitable for preparing laminates of polyvinyl chloride or polyvinyl chloride copolymers with wood or metal are prepared from a three component polymer mixture of polyvinyl chloride or its copolymers itself, a direct ethylene copolymer and another direct ethylene copolymer containing units derived from a monomer containing glycidyl units. The adhesive is especially useful for preparing PVC/wood laminates which are subject to outdoor weathering.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No: 07/904,725 filed Jun. 26, 1992 now U.S. Pat. No. 5,209,983.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to adhesive compositions suitable for laminatingpolyvinyl chloride to wood or other substrates. The adhesives are blendsof polyvinyl chloride and two different ethylene copolymers. Theadhesive compositions are useful for preparing laminates which aresuitable for outdoor use.

2. Description of Related Art

Adhesives are used in a wide variety of fields. For some applicationsthe requirements of the adhesive may be very demanding. Not only mustthe composition provide good initial adhesion, but the adhesive bond maybe subject to extremes of temperature or environment. Such is the casefor adhesives used in outdoor building applications.

In the building industry, siding may be a laminate structure whichrequires an adhesive to make that laminate. That adhesive should bereadily applied to the laminate components, it must adhere well to eachof the layers of the laminate and the bonding must withstand the rigorsof outdoor use. Common among siding materials are wood, metals andpolyvinyl chloride.

Various materials are known for use as thermoplastic adhesives which maybe applied in the melt as a molten fluid. If the viscosity of the meltis suitable for film extrusion, and cooled extruded film is stiff enoughto handle, these copolymers may be adapted to films for laminatingadhesives.

Various ethylene copolymers are well known as hot-melt thermoplasticadhesives. Ethylene/alkyl acrylate/carbon monoxide copolymers have beendescribed for use as adhesives for use with a wide variety of substratesin PCT publication WO91/18043.

Ethylene vinyl acetate copolymers have long been known for use ashot-melt adhesives.

Ethylene butyl acrylate copolymers for use as hot-melt adhesives havebeen described in U.S. Pat. No. 4,816,306.

Ethylene/alkyl acrylate or vinyl acetate/carbon monoxide terpolymershave been disclosed for use especially as plasticizers for PVC in U.S.Pat. No. 3,780,140. Blends with PVC can be used as limp or rigid films.The terpolymers are disclosed alternately as being blendable with alarge number of other materials, including epoxy resins.

None of the references disclose the particular three componentcombination of this invention, nor is there even any suggestion thatblends of PVC with any ethylene copolymer or copolymers would besuitable as laminating adhesives.

There is a need for an adhesive material which provides good adhesionbetween polyvinyl chloride sheeting and various substrates, particularlywood, and which maintains that adhesion under outdoor weatheringconditions.

SUMMARY OF THE INVENTION

The present invention provides for a thermoplastic adhesive forpolyvinyl chloride (PVC) or its copolymers which is a blend of PVCitself or PVC copolymers with two distinct ethylene copolymer types, oneof which contains a glycidyl group.

According to the present invention, there is provided an adhesive forPVC, comprising:

a) 5-56 weight percent of polyvinyl chloride or a polyvinyl chloridecopolymer;

b) 15-75 weight percent of a first ethylene copolymer, said firstcopolymer being a direct copolymer containing 25-50 weight percentethylene and from 75-50 weight percent vinyl acetate;

c) 2-25 weight percent of a second ethylene copolymer, said secondcopolymer being a direct copolymer containing from 0-50 weight percentof a moiety derived from at least one alkyl acrylate, alkylmethacrylate, alkyl vinyl ether, vinyl acetate or mixtures thereof wherethe alkyl radical contains 1-8 carbon atoms, and also containing 1-15weight percent of a moiety derived from glycidyl acrylate, glycidylmethacrylate or glycidyl vinyl ether.

The first ethylene copolymer should not contain any glycidyl monomerunits.

DETAILED DESCRIPTION OF THE INVENTION

In this disclosure, the word copolymer is used to embrace polymers withtwo or more monomers. If a distinction is necessary, the words`bipolymer`, `terpolymer` etc. are used. The phrase `direct copolymer`describes a copolymer made by polymerizing monomers together directly,rather than by adding a polymer chain onto an existing one by graftingon its monomer.

This invention concerns adhesives useful particularly for preparinglaminates of polyvinyl chloride or copolymers to substrates such as woodor metal, particularly wood. Such adhesives will also be useful in otherapplications where adhesion to PVC and metal or wood is needed. Theadhesive is thermoplastic, and is especially useful in the form of afilm laminating adhesive.

Better adhesives generally have stronger adhesive bonds. Preferably thebonds should be as strong as the substrate so that failure is limitedonly by the cohesive strength of the substrate layers. If not as strongas the strength of the layers, the bond should at least be strong enoughto hold up under use conditions.

It has now been found that superior adhesives for the present purposeare obtained by blending one type of ethylene copolymer with some PVCitself or PVC copolymer, together with a second ethylene copolymer typewhich contains a glycidyl group. The blend remain thermoplastic andmelt-processible into films suitable for use as laminating adhesives.

It appears that all three components are necessary for this invention.Without limiting ourselves to a particular explanation, each componentmay especially help in a different way. Thus, while PVC obviouslyadheres to itself and hence enhances adhesion to the PVC film side ofthe laminate, it can provide versatility in achieving the best stiffnessfor a film laminate. Alone, PVC does not adhere to wood or othersubstrates well. The two ethylene copolymers are by contrast veryflexible and so balance the laminate adhesive stiffness. Both may servedifferent adhesive functions. Thus, the glycidyl group reactivelyadheres to polar groups among others, and is essential to provideadhesion under moist conditions. However, the glycidyl containingethylene copolymer is not very compatible with PVC and could not be usedalone with PVC. The other ethylene copolymer, may serve acompatibilizing as well as an adhesive function.

The PVC in the blend may be the same PVC as the layer to be bonded or itmay be another grade or a PVC copolymer. Typical copolymers suitable forthis invention are copolymers of vinyl chloride with vinyl acetate,propylene, alkyl acrylate, vinylidene chloride, diethyl fumarate ordiethyl maleate. PVC copolymers which have a lower melting point thanthe PVC to be laminated may be preferable. While the three-componentblend adhesive softens at a lower temperature than the PVC itself, lowermelting point PVC polymers may lower the temperature at which theadhesive blend may be prepared and applied.

The first ethylene copolymer is a copolymer of 15-75 weight percentethylene and one, two or more monomers not including a glycidyl monomer.It is chosen so that it is essentially miscible with PVC. It will alsobe miscible with the second ethylene copolymer containing a glycidylgroup. The other monomer consists of vinyl acetate or methyl acrylate.Terpolymers containing both comonomers are suitable. The particularcomonomer and its amount must provide enough polarity for the copolymerto be miscible with PVC. The preferred copolymer is ethylene/vinylacetate, and the preferred ethylene/vinyl acetate copolymer contains55-70 weight percent vinyl acetate. Carbon monoxide and sulfur dioxide,preferably together with an alkyl acrylate, can also provide thispolarity and this is the subject matter of copending application Ser.No. 07/904,725. Compatible terpolymers include ethylene/n-butylacrylate/carbon monoxide or ethylene/ethyl acrylate/carbon monoxide.

The first ethylene copolymer should not contain any glycidyl monomerderived units, though small amounts of other monomers which do notaffect the overall function of the copolymer in the adhesive arepossible.

The second ethylene copolymer may have a comonomer which is selectedfrom alkyl acrylates, methacrylates and vinyl acetate, and may bypresent in amounts up to about 50 weight percent. The second copolymermust have a comonomer containing a glycidyl group however. Suchcomonomers are glycidyl acrylate, glycidyl methacrylate and glycidylvinyl ether. This monomer may be present at levels between 1 and 15weight percent, preferably from 3-11 weight percent. The preferredsecond copolymers are terpolymers, and the preferred terpolymer isethylene/n-butyl acrylate/glycidyl methacrylate containing from 10-40percent n-butyl acrylate and 3-11 percent glycidyl methacrylate. Smallamounts of other monomers which do not affect the overall function ofthis monomer in the adhesive composition are possible.

Both first and second ethylene copolymers are direct copolymers. Theymay be prepared by well known methods of free-radical, high-pressurepolymerization. Preparative methods are given in U.S. Pat. Nos.3,780,140 and 4,497,941. Both patents are herein incorporated byreference.

The amount of each of the three components of the adhesive can varywidely. The amount of PVC or PVC copolymer is designed to give the mostsuitable level of flexibility for the laminating adhesive for particularsubstrates, as well as the best balance of in-use adhesive behavior.

Very generally, more of the first ethylene copolymer than secondethylene copolymer is preferred. However, because adhesion is such acomplex phenomenon, and particularly with regard to its permanence underend-use conditions, within the broad limits specified, wide variation ispossible. It is within the skill of the artisan to vary levels to bemost suitable for a particular environment and laminate structure.

In making a laminate of PVC or PVC copolymer with a wood or metallaminate co-layer, it is preferable to pre-treat the wood or metalsubstrate with a silane solution. This is a well known procedure and isparticularly useful when the laminates are subject to a moistenvironment. Typical silanes for this purpose are amino-functionalsilanes, particularly amino trialkoxy silanes, for exampleaminotriethoxy silane.

Unlike typical hot-melt adhesives, this three-polymer-component blendadhesive does not require plasticizers, tackifiers, waxes and othercommon formulating additives which are often required to make adhesiveformulations. The adhesive of this invention, however, is preferablyused as a film, i.e. as a laminating adhesive, rather than being appliedas a hot-melt, and as such requires sufficient `body` to be used as afilm at room temperatures. Small amounts of formulating additives,however, may be present to the extent that they do not detract from theoverall utility of the adhesive. A formulated more fluid version of thethree polymer adhesive blend however, applied as a hot-melt adhesive ispossible.

On a large scale, it is possible to prepare the three component blendadhesive using typical extrusion equipment. Standard considerations withregard to adequate mixing and best blending the different viscositycomponents will be taken. In the laboratory, the three polymer componentblend is preferably prepared by a `masterbatch` and `letdown` procedure.PVC is relatively stiff and high melting, while the ethylene copolymersare quite soft and lower melting. Blending all three components at thesame time in the required amounts is possible, but it is easier togradually `soften` the PVC with some of the first copolymer, lateradding the softened PVC/first copolymer masterbatch blend to the secondethylene copolymer, and if necessary more first ethylene copolymer, toachieve the final required amount of each component.

LABORATORY PREPARATION OF ADHESIVE BLENDS

PVC stabilizers which find standard use with PVC are added first. In thelaboratory this may be done using a high speed Welex mixer to absorb theliquid stabilizers. The stabilizers are added to dry PVC in the mixerwhich is allowed to rotate at about 2500 RPM until the stabilizers areadsorbed, the temperature being allowed to rise to about 80° C. Theblend is then cooled to about 60° C. with lower RPM and jacket coolingwater, and then discharged from the mixer as a powder.

For blends containing more than 50 weight percent total ethylenecopolymer a 50/50 masterbatch blend of PVC/first ethylene copolymer ismade keeping the mixture PVC rich until fusion occurs. This blend canthen be diluted with additional first and second ethylene copolymeruntil the desired blend ratio is obtained. To make the masterbatch, aBrabender or Haake high shear mixer is used. For the examples a batchsize of about 280 g. was prepared. The temperature is set at 175° C.,and the mixer set to have a rotation speed of about 100 RPM. Themasterbatch is mixed, starting PVC rich, for about 5 minutes, then theremainder of the first copolymer is added, continuing mixing for another10 minutes while the temperature is allowed to rise to just under 200°C. A gel-free blend is obtained. This masterbatch is then used withadditional amounts of second and first ethylene copolymer as required toobtain the desired blend. In this `letdown` step a smaller Brabendermixer is used, preparing batches of about 50 g. The temperature of themixer is set at 180° C. and 100 RPM also. This mix is allowed to run forabout 7 minutes and a maximum temperature of about 160° C. Gel-freeblends are obtained. Scale up for commercial size preparations will bereadily apparent from these lab scale preparative details.

LABORATORY LAMINATE PREPARATION PROCEDURE

Sheets of the adhesive being about 5 mil. thick are prepared in aconventional laboratory press using a temperature of about 180 ° C.,cooling under pressure.

PVC/adhesive/wood laminates using the sheet adhesive are prepared asfollows. A 2 inch×4 inch plywood strip is first cut. It is optionallytreated with a silane solution. This is done by coating with a 0.4weight percent solution of silane A1100 (aminotriethoxy silanemanufacture by Union Carbide Corp.) in 50/50 water/isopropanol. Thesolution is then allowed to dry. An assembly of the plywood, filmadhesive and PVC sheet (B. F. Goodrich `Duracap` which is a semi-rigidvinyl sheet) is made. A strip of TEFLON film is inserted at the end ofthe assembly between the adhesive film and the PVC to prevent adhesion,so that when the laminate is placed in tensile tester, an unadheredportion of the PVC can be gripped for tensile testing. The test assemblyis placed in a vacuum laminator which consists of a 1/4"×9"×19" steelbase plate having 1/4" pipe connections for applying vacuum at each end.The plate edges contain a soft pliable sealant to seal the edges ofKAPTON 5 mil. sheet which is used as a cover. A 3/8"×3/8" steel strip isplaced over the KAPTON film on the sealing edge of the laminator andclamped evenly. The assembly is placed into a cool standard laboratorypress and the press closed to just above the assembly without anypressure on it. Vacuum is applied to each pipe connection to pull thetop sheet down firmly on the test assembly. The press is then heated to180° C. over a period of about 7 minutes. It is found that using thistime frame, the PVC, adhesive and wood is heated to a suitabletemperature for adhesion, but does not reach the press temperature whichwould melt the PVC. Under different conditions, times will vary. Theprinciple is to select a time and temperature to provide sufficientadhesion, yet not enough to allow the integrity of the vinyl film to bealtered. The temperature reached by the adhesive is thus somewhat lessthan that which would distort the vinyl film. The heat is then turnedoff and allowed to cool to room temperature using cooling water. Thevacuum is released and the laminated test sample removed from thelaminator.

LAMINATE TESTING PROCEDURE

Laminates were tested as follows. The sample is placed in an InstronTensile tester so that the PVC can be peeled from the wood substrate atan angle of 180°. The cross head of the tensile tester is run at 0.2inches per minute. The average load and if possible the initial loadrequired to peel the PVC is recorded and the mode of failure noted. Themodes of failure may be (i) adhesive failure at the PVC adhesiveinterface, referred to the table as AD/PVC; (ii) adhesive failure at thewood adhesive interface--AD/WOOD and (iii) cohesive failure of theadhesive or even of the wood--COH. Where failure is both cohesive andadhesive, it is often difficult to discern the various effects. This isreferred to as COH,AD.

PERFORMANCE TESTING

Several adhesive blends were made as described, used for preparation oflaminates, and subjected to testing to determine the peel strength ofthe PVC/wood bond. The compositions and results are shown in Table I. Ina second series of tests, several of the same adhesive blends were usedbut the laminates were subjected to a water soak to simulate a wetenvironment. The laminates were submerged in de-ionized water at 60° C.for about 24 hours. The samples were removed and dried at 60° C. undervacuum overnight. The samples were then subjected to peel strength testsin the same manner as for the UN-soaked laminates. In the samplessubject to the water-soak, laminates were made in exactly the samemanner as for laminates tested without a water-soak except that somelaminates were made without silane treating the wood. This was designedto see how significant silane treatment was in maintaining laminatestrength.

                  TABLE 1                                                         ______________________________________                                        STRENGTH OF PVC/WOOD LAMINATES                                                                                PEEL                                                                          STR.                                          SAM-  PVC     COPOL 1   COPOL 2 lbs/   FAIL                                   PLE   WT %    WT %      WT %    inch   MODE                                   ______________________________________                                        A1S   24      67        9       12.5   AD/PVC                                 B1S   19      71.3      9.7     12     AD/PVC                                 C1S   14      75.7      10.3    10.5   AD/PVC                                 D1S   0       87.8      12.2           BREAK                                  D1S*  SAME AS D1S             7.5    AD/PVC                                   E1S   20.4    60.6      19      11.25  AD/PVC                                 F1S   22.2    63.8      14      11.7   AD/PVC                                 G1S   25.4    69.6      5       12     COH                                    H1S   26.5    71.5      2       13.25  AD/                                                                           WOOD                                   I1S   27.2    72.8      0       12.5   COH                                    J1S   38.6    47        14.4    3.7    AD/PVC                                                                        &WOOD                                  K1S   45.8    37        17.2    4.7    COH,AD                                 L1S   53.1    27        19.9    12.7   COH,AD                                 M1S   25.4    69.6      5       6.5    COH                                                  E/VA                                                            ______________________________________                                         All samples have had wood treated with silane.                                *Repeat.                                                                      Sample codes such as C1S designate the laminate made with a given adhesiv     composition. The adhesive composition is C, 1 refers to the test conditio     (UNsoaked) and S means a silane treatment of the wood.                        PVC is GEON30 (B. F. Goodrich) for Samples A1S through L1S and for Sample     M1S is a PVC copolymer with vinyl acetate comonomer and a m.p. of 130 deg     C., Grade KYHH mfg. by KeysorCentury Corp.                                    The PVC or PVC copolymer is first stabilized as described with 3.0% Mark      4710, a Barium/Zinc stabilizer manufactured by Witco div. of Argus Corp.      and 5.0% ESO DRAPEX 6.8, an epoxidized soybean oil also manufactured by       Witco.                                                                        COPOL 1 for Samples A1S through L1S is an ethylene/nbutyl acrylate/carbon     monoxide terpolymer, MI 100, 63/27/10 weight ratio. For Sample M1S it is      an ethylene/vinyl acetate copolymer containing 63 weight % vinyl acetate.     COPOL 2 for Samples A1S through L1S is an ethylene/nbutyl                     acrylate/glycidyl methacrylate terpolymer, MI 10.6, 63.6/28/8.4 weight        ratio, and for Sample M1S is a terpolymer containing the same monomers, M     12, 66.75/28/5.25 weight ratio.                                               Peel Strength values quoted actually represent a true peel strength where     adhesive failure occurs. They represent some composite value of adhesive      and cohesive strength where failure is not purely adhesive.              

                  TABLE II                                                        ______________________________________                                        STRENGTH OF WATER-SOAKED                                                      PVC/WOOD LAMINATES                                                                                            AD-                                           SAMPLE  PVC     COPOL1   COPOL2 HESION                                        MODE    WT %    WT %     WT %   lbs/inch                                                                             FAIL                                   ______________________________________                                        C2      SAME AS C         4.8/2.75 COH,AD/                                                                       PVC&                                                                          WOOD                                       C2S     SAME AS C         7.8/6.0  COH,AD/                                                                       PVC&                                                                          WOOD                                       D2S     SAME AS D         6.2/6.2  AD/PVC                                     E2      SAME AS E         6.0/1.2  COH,AD/                                                                       PVC&                                                                          WOOD                                       E2S     SAME AS E         8.5/8.5  AD/PVC                                     L2      SAME AS L         5.2/1.0  COH,AD/                                                                       PVC&                                                                          WOOD                                       L2S     SAME AS L         8.0/8.0  AD/PVC                                     G2      SAME AS G         1.6/1.6  COH,AD/                                                                       PVC&                                                                          WOOD                                       G2S     SAME AS G         3.9/3.3  COH,AD/                                                                       PVC&                                                                          WOOD                                       I2      SAME AS I         1.1/1.1  COH,AD/                                                                       PVC&                                                                          WOOD                                       I2S     SAME AS I         1.9/1.9  COH,AD/                                                                       WOOD                                       M2S     SAME AS M         7.0/6.0  COH                                        ______________________________________                                         Compositions are given in Table 1. The number 2 refers to the second          condition of laminate testing, after a watersoak.                             Peel Strength values represent an initial strength, followed by a lower       average value on continued peeling                                       

The data in Table 1 show that for a range of compositions containing thethree components, strength of the laminates is high. Values above 3.0are considered good. In many cases the adhesive bond exceeded thecohesive strength of the adhesive, and so failure occurred within theadhesive. Even when failure was at the PVC/adhesive or wood/adhesiveinterface however, that is to say adhesive failure, adhesive strengthwas high. Values are quite variable, probably because there are threepossible modes of failure, or combinations of these, as discussed.Because of the variability in the data obtained it is difficult to saywhich composition represents a preferred one. It is currently believedthat in end-use conditions, compositions which give only cohesivefailure may be the preferred ones, though no data in actual end useconditions over a period of time are available.

Laminate D1S, which contained no PVC in the adhesive D, broke in oneinstance and gave no measurable strength. A repeat gave a strength of7.5 lbs per inch. Although this value might be adequate, it is believedthat when no PVC is present the adhesive bond may be unreliable, inaddition to providing a film which is very flexible and difficult tohandle. Compositions J1S and K1S gave adequate but relatively lowstrength values. Separate experiments suggest that in this triplecomponent blend there are composition regions where miscibility of thethree components is less. Such differences could explain the lowervalues.

While laminate I1S, containing no glycidyl containing copolymer in theadhesive blend, manifested a high strength of 12.5, its unsuitabilityshowed up when subjected to a water-soak. Values in Table 2 are forlaminates which have been soaked in water. Values for initial peelstrength and also the average peel strength on continued peeling weredetermined. All strength values are much lower than when tested dry.This is because the strength of the wood is severly lowered by the watersoak, so the values are strongly affected by this weakness. The initialvalues are probably more significant than values on continued peel,since this represents the threshold valued needed to cause adhesivefailure.

In all cases, silane treatment of the wood gives higher failure values.As a result, it is preferred to pre-treat wood laminates when thelaminates will be subject to a moist environment. Samples in Table 2containing adhesive component I give very low values whether silanetreated or not. This is the only sample which contained no glycidylcontaining ethylene copolymer. It is clear that this component is neededwhen the laminate is subjected to a moist environment.

Sample M1S and M2S are for ethylene/vinyl acetate, having a high levelof vinyl acetate, as Copolymer 1. Also in those samples Copolymer 2 hasa lower level of glycidyl methacrylate and the PVC is a PVC copolymerhaving a lower melting point of about 130 compared with about 170 forGeon PVC. Results appear satisfactory, particularly after a water-soak.The results are not directly comparable with the rest of the samples inwhich Copolymer 1 is a terpolymer containing only 37 weight percenttotal comonomer including 10 weight percent carbon monoxide, since bothCopolymer 2 and the PVC is different. However, it is clear thatethylene/vinyl acetate with a high level of vinyl acetate, which is acopolymer compatible with PVC, is quite satisfactory.

We claim:
 1. An adhesive for polyvinyl chloride and polyvinyl chloridecopolymers, comprising:a) 5-65 weight percent of polyvinyl chloride or apolyvinyl chloride copolymer; b) 15-75 weight percent of a firstethylene copolymer, said first copolymer being a direct copolymercontaining 25-50 weight percent units derived from ethylene and 75-50weight percent from vinyl acetate; c) 2-25 weight percent of a seconddirect ethylene copolymer containing from 0-50 weight percent unitsderived from a comonomer selected from the group consisting of alkylacrylates, alkyl methacrylates, alkyl vinyl ether, and vinyl acetate, ormixtures thereof, where the alkyl radical contains 1-8 carbon atoms, andalso containing 1-15 weight percent units derived from glycidylacrylate, glycidyl methacrylate or glycidyl vinyl ether.
 2. The adhesiveof claim 1 wherein the second ethylene copolymer is ethylene/n-butylacrylate/glycidyl methacrylate.
 3. The adhesive of claim 2 wherein thesecond ethylene copolymer contains 10-40 weight percent units derivedfrom n-butyl acrylate and 3-11 weight percent from glycidylmethacrylate.
 4. The adhesive of claim 3 wherein the first ethylenecopolymer contains 55-70 weight percent units derived from vinylacetate.
 5. A laminate comprising a polyvinyl chloride or a polyvinylchloride copolymer layer, and a second layer, wherein the two layers areadhered with the adhesive of claim
 1. 6. A laminate comprising apolyvinyl chloride or a polyvinyl chloride copolymer layer, and a secondlayer, wherein the two layers are adhered with the adhesive of claim 3.7. The laminate of claim 5 wherein the second layer is wood.